The present invention relates generally to rotary atomizers and more particularly to a rotary atomizer having improved performance for particulate paints.
Currently, many paints are applied by rotary atomizers to work pieces, such as automobile bodies. Rotary atomizers include a rotating bell cup having a generally conical overflow surface between a radially inward central axial opening and a radially outward atomizing edge. At or near the atomizing edge, the angle of the overflow surface relative to the axis of the bell cup decreases sharply to form a lip adjacent the atomizing edge. The purpose of this lip is to generally direct the atomized paint more axially forward and reduce radial scatter. The known atomizer bell cups further include a deflector, also of generally rotational symmetry, disposed in front of the central axial opening. Paint entering the bell cup through the central axial opening contacts the rear surface of the deflector and is disbursed radially outwardly towards the overflow surface.
In the known atomizer bell cups, the paint follows a tortuous, turbulent path from the nozzle to the atomizing edge. As a result, the paint flow to the atomizing edge is turbulent and fluctuates cyclically. As a result, paint from the atomizer is atomized to a wide variety of paint droplet sizes. The paint droplets can vary by up to 100 microns or more.
Current rotary atomizers are unable to obtain good color matching applying paints with particulates, such as mica. Generally, the mica comprise particles on the order of 3 microns by 200 microns. When this paint is applied by rotary atomizers, the mica particles are oriented generally perpendicular to the application surface. As a result, the paint has a different tint or color than intended, i.e. with the mica particles laying flat. In order to correct this problem, a second coat of the paint is typically applied with air atomized spray guns rather than rotary atomizers. This second coat provides the proper color; however, air atomized spray guns have a low transfer efficiency (approximately 50%) compared to rotary atomizers (approximately 80%). The air atomized spray guns therefore increase the amount of paint lost, increasing the cost of the paint process and cause environmental concerns regarding the disposal of the lost paint.
The present invention provides a rotary atomizer which provides improved color matching. Generally, the improved atomizer provides a more uniformed paint droplet size, which in turn facilitates control of the particulates in order to assure proper orientation of the particulates and obtain good color matching.
The rotary atomizer bell cup according to the present invention provides several inventive features directed toward reducing deviation in paint droplet size. First, the bell cup includes a generally conical overflow surface having a generally constant flow angle between a deflector and the atomizing edge. Further, the exposed surface area of the overflow surface is increased by decreasing the size of the deflector relative to previous bell cups in order to cause evaporation of solvent from the paint from the overflow surface. The diameter of the atomizing edge is also increased, thereby reducing the thickness of the paint film at the atomizing edge. The bell cup is designed to reduce flow deviations of the paint as it travels from the axial opening to the spray edge in order to provide laminar flow of the paint across the overflow surface and the atomizing edge.
The bell cup is made hollow in order to reduce the weight of the bell cup. A rear cover is secured to the rear of the bell cup body, enclosing an annular cavity.